Conventionally, various backlight correction techniques have been used in camera devices. As one of the backlight correction techniques, there is known, for example, a method for expanding a dynamic range by using an image pickup element in which a long exposure signal (video signal having a long exposure time) and a short exposure signal (video signal having a short exposure time) are output within one field period (see, for example, Patent Literature 1). In general, in a conventional method, knee processing for compressing a signal at the luminance level of a knee point (predetermined luminance level) or higher is performed in order to widen a dynamic range. After the knee-processed long exposure signal and short exposure signal are synthesized, a tone correction is made to the synthesized signal to output a final video signal. Note that the synthesis of the long exposure signal and the short exposure signal is performed so that the long exposure signal is used for luminance levels lower than the predetermined luminance level (synthesis point), and the short exposure signal is used for the luminance level of the synthesis point or higher.
In commonly-used knee processing, a knee point is a fixed value. In that case, there has been the problem that it is difficult to smoothly synthesize the long exposure signal and the short exposure signal, thus causing a sense of gradation to deteriorate. Hence, there has been conventionally proposed a technique to perform control, so as to lower the knee point of a long exposure signal and raise the gain of a short exposure signal, only when a subject (extremely bright subject) for which a dynamic range needs to be expanded is photographed.
FIG. 6 is an explanatory drawing of knee point control in conventional knee processing. As illustrated in FIG. 6(a), knee processing for compressing signals at the luminance level of a knee point or higher is performed on a long exposure signal when a bright subject is photographed. On the other hand, as illustrated in FIG. 6(b), a knee point adjustment is made so as to lower the knee point when a brighter subject is photographed. Consequently, it is possible to obtain an image improved to some degree in the sense of gradation.
However, simply lowering the knee point of a long exposure signal as in the conventional method is not sufficient in terms of improvement in the sense of gradation. That is, since an inclination at the synthesis point of the long exposure signal and the short exposure signal (inclination of an incident light amount-luminance level line representing a variation of the luminance level with the incident light amount) differs, an image taken results in a weakly-contrasted image. In addition, in the conventional method, the long exposure signal and the short exposure signal differ in inclination at such a comparatively high-brightness part (bright part within an image) as the synthesis point of the long exposure signal and the short exposure signal. This gives the impression that gradational variation is not smooth (even in the case of such a degree of shift in gradational variation as not to be concerned about in dark parts within a screen).